|  | 1.1 | The OSI Reference Model and the Problems it Solves
| 1.1.4 | Encapsulation
How does Layer 4 in Host B know what Layer 4 in Host A wants? Layer 4's specific requests are stored as control information, which is passed between peer layers in a header block that is attached to the actual application information. Each layer depends on the service function of the OSI reference model layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then, it can add whatever headers and trailers the layer will use to perform its function. The concept of a header and data is relative, depending on the layer currently analyzing the information unit. For example, to Layer 3, an information unit consists of a Layer 3 header and the data that follows. Layer 3's data, however, can potentially contain headers from Layers 4, 5, 6, and 7. Further, Layer 3's header is simply data to Layer 2. This concept is illustrated in the Figure. Finally, not all layers need to append headers. Some layers simply perform a transformation on the actual data they receive to make the data readable to their adjacent layers. For example, the network layer provides a service to the transport layer, and the transport layer presents data to the network layer. The network layer then encapsulates the data within a header. This header contains information required to complete the transfer, such as source and destination logical addresses. The data link layer, in turn, provides a service to the network layer encapsulating the network layer information in a frame. The frame header contains information required to complete the data link functions. For example, the frame header contains physical addresses. The physical layer also provides a service to the data link layer by encoding the data link frame into a pattern of ones and zeros for transmission on the medium. For example, let's assume that Host A wants to send the following e-mail to Host B: The small gray cat ran up the wall to try to catch the red bird. Five conversion steps occur during data encapsulation, which enables the transmission of the e-mail to the appropriate destination: Step 1 As a user sends an e-mail message, its alphanumeric characters are converted to data, starting at Layer 7 down through Layer 5, and are sent over the network. Step 2 By using segments at Layer 4, the transport function packages data for the network transport and ensures that the message hosts at both ends of the e-mail system can reliably communicate. Step 3 The data is placed into a packet (or datagram) at Layer 3 that contains a network header with source and destination logical addresses. Then, the network devices send the packets across the network along a chosen path. Step 4 Each network device must put the packet into a frame at Layer 2. The frame allows connection to the next directly connected network device on the link. Each device in the chosen network path requires framing to connect to the next device. Step 5 The frame must be converted into a pattern of ones and zeros for transmission on the medium (often copper wire or optical fiber) at Layer 1. A clocking function enables the devices to distinguish these bits as they traverse the medium. The medium on the physical network can vary along the path used. For example, the e-mail message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN.
| Web Links
| Cisco Connection Online
| Cisco Documentation
| Search Cisco